Radiant energy imaging with scanning pencil beam

ABSTRACT

A pencil beam of X-rays scans an object along a line of direction before an X-ray detector to produce an image of the line along a picture tube. By relatively displacing the object scanned and the line of scan in a direction transverse to the line of scan, a sequence of lines appear on the display to produce an image of concealed objects, such as guns.

United States Patent [191 Stein et a1.

[ RADIANT ENERGY IMAGING WITH SCANNING PENCIL BEAM [75 Inventors: Jay A.Stein, Framingham; Roderick Swift, Belmont, both of Mass.

[7 3] Assignee: American Science & Engineering Inc., Cambridge, Mass.

[22] Filed: July 7, 1971 [21] Appl. No.1 160,363

[52] US Cl. ..250/363 51 Int. Cl. 0011'1/20 [58] Field of Search 250/715R, 71.5 S, 250/77, 83.30, 105, 52

[56] References Cited UNITED STATES PATENTS 3,106,640 10/1963 Oldendorf250/833 D X 1i] 3,780,291 [451 Dec. 18, 1973 Wilson, Jr 250/105 Jordan250/715 R Primary Examiner-James W. Lawrence Assistant Examiner-Davis L.Willis Attorney-Charles Hieken I 5 7 I ABSTRACT A pencil beam of X-raysscans an object along a line of direction before an X-ray detector toproduce an image of the line along a picture tube. By relativelydisplacing the object scanned and the line of scan in a directiontransverse to the line of scan, a sequence of lines appear on thedisplay to produce an image of concealed objects, such as guns.

12 Claims, 2 Drawing Figures vmnmnmm ma 3.780.291

INVENTORS JAY A. STEIN RODERICK SWIFT ATTORNEYS RADIANT ENERGY IMAGINGWITH SCANNING PENCIL BEAM BACKGROUND or THE INVENTION The presentinvention relates in general to radiant energy imaging and moreparticularly concerns novel apparatus and techniques for displaying avisual image of concealed objects with sufficient resolution to identifythe object while keeping the intensity of radiation relatively low. Thesystem is reliable, relatively economical and may be operated byrelatively unskilled personnel.

The problem of detecting contraband concealed in packages and onpersonsis a serious one. X-ray equip ment is useful for assisting in thediscovery of con 1 cealed contraband. Conventional X-ray equipment iscostly, requires operation by skilled personnel and may well subjectpersonnel and parcels to undesired excessive dangerous radiation.

Accordingly, it is an important object of this invention to provide anX-ray imaging system that overcomes one or more disadvantages ofconventional systerns.

It is an important object of this invention to provide an X-ray imagingsystem for displaying an image of concealed devices without exposingpersonnel or parcels to excessive radiation.

It is a further object of the invention to achieve one or more of thepreceding objects with apparatus that is relatively inexpensive andcapable of being operated by relatively unskilled personnel.

It is a further object of the invention to achieve one or more of thepreceding objects with apparatus that operates reliably and isrelatively easy to manufacture.

SUMMARY OF THE INVENTION According to the invention, there is means forscanning a radiation sensitive detector along a curve with a pencil beamof radiation to provide a line image signal characteristic of radiantenergy response between the source of the pencil beam and the radiationsensitive detector, and means for displaying the image represented bythe image signal. The radiation sensitive detector and the source are infixed relationship. The detector may be positioned for receiving directand/or reflected or scattered radiation. Preferably there is means forrelatively displacing the curve scanned and an object to produce asequence of image signals representative of the radiant energy responseof the object in two dimensions. There is means for relativelydisplacing the region embracing the object and an assembly comprisingthe source and radiation sensitive detector or detecting means toestablish relative translating motion in a direction transverse to aline joining the source and the detecting means. Preferably the curve isa line with the relative displacement between object and line being in adirection orthogonal to the line. Preferably the detec'tor comprises asodium iodide or cesium iodide crystal that produces a visiblemanifestation of the intensity of the incident radiation that may besensed by a photodetector to provide a characteristic electrical outputsignal that may be applied to a television display system that mayincorporate a storage tube.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a pictorial representation ofa parcel inspection system according to the invention; and

FIG. 2 is a pictorial representation of an exemplary embodiment of theinvention for inspecting personnel.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to thedrawing and more particularly FIG. 1 thereof, there is shown a pictorialrepresentation of a system according to the invention for scanningparcels. A parcel I1 is scanned by the invention to produce an image 12of contraband on the video storage and display unit 13. An X-ray tube 14provides a generally conical beam of X-rays that are collimated into afan beam 16 by slit collimator l7 oriented generally vertically as shownand incident upon the rotating collimation disc 18 formed with an arrayof peripheral radial slits, such as 21, for intercepting fan beam 16 toproduce pencil beam 23. Pencil beam 23 scans parcel II and radiationsensitive detector 25 from top to bottom as rotating disc 18 rotates inthe direction of arrow 24 to provide an image signal over output line 26that is transmitted to video storage and display unit 13 to produce theimage 12 of the parcel scanned as conveyor 27 carries parcel 11 in thedirection of arrow 28 across the line being scanned.

Since most specific elements of the system are known to those skilled inthe art who can practice the invention from an examination of FIG. I andthe accompanying description, minute specific details are omitted so asto avoid obscuring the invention.

The geometry and timing of the system is arranged so that each slit 21causes a new pencil beam to strike the top of detector 25 just after theprevious pencil beam has swept past the bottom of the detector. That isto say, the height of fan beam 16 corresponds substantially to theseparation between adjacent ones of slits 21 at substantially themaximum radial distance from the edge of disc 18 where the slitsintercept fan beam 16. While FIG. I shows the elements that provide thescanning pencil beam source in exploded form to better illustrate theprinciples of the invention, the elements l4, l7 and 18 are preferablyhoused relatively close together in an enclosure that shields radiationso that the only significant radiant energy that escapes is that inpencil beam 23.

As parcel 11 moves past the line being scanned, it differentiallyattenuates the X-rays in. pencil beam 23 inci dent upon detector 25 sothat the electrical signal provided on output line 26 is amplitudemodulated in proportion to the instantaneous X-ray flux incident uponit. This signal thus corresponds to a vertical line image of thetransmissivity of parcel 11 and is analogous to one scan line of atelevision video signal. As parcel 11 moves horizontally past the linebeing scanned, sequential pencil beams intercept slightly displacedregions of parcel 11 so that the corresponding electrical signals fromdetector 25 may be appropriately displayed lineby-line to produce a twodimensional image of parcel 11 in X-rays analogous to the display of apicture on a television monitor as formed by line-by-line images. Theoutput of detector 25 may thus be processed in ac cordance with the samestorage and display techniques used in conventional video systems tostore and display single raster images. Since these techniques are wellknown in the art, further discussion of them is unnecessary here.

Although detector 25 is shown behind the object being scanned forresponding to the radiant energy transmitted through the object beingscanned, it is within the principles of the invention to position thedetector in the region between the radiant energy source and the objectbeing scanned to respond to the scattered energy. This arrangement helpsthe apparatus detect concealed objects having different scatteringcharacteristics from their surroundings. Moreover, a system according tothe invention may include both detecting means before and behind theobject being scanned for simultaneously providing signals representativeof both radiant energy transmission and scattering. Appropriatelycombining such signals may help increase the ability of the system todetect a wide variety of con cealed objects.

Referring to FIG. 2, there is shown a pictorial diagram illustrating thelogical arrangement of a system according to the invention for personnelinspection. This system embodies the principles of the system of FIG. 1;however, the pencil beam scans horizontally, and the scanning system andperson relatively move vertically to produce a two dimensional image ofthe person. Like elements in the system of FIG. 2 are designated bycorresponding reference numerals.

A vertically movable platform 41 supports the pencil beam sourcecomprising X-ray tube 14, fan beam collimator l7 and rotatingcollimation disc 18 to scan per son 42 along a sequence of horizontallines as detector 25 and platform 41 move down together. Detector 25 isalso'supported for vertical displacement.

Details of specific means for vertically displacing detector 25 andplatform 41 are well within the skill of one having ordinary skill inthis art and are omitted so as to avoid obscuring the principles of theinvention. They might, for example, be guided by vertical shafts, atleast one of which was a rotating feedscrew supporting detector 25 andplatform 41 rotating in synchronism so that platform 41 and detector 25move together. Numerous other techniques could be employed for effectingvertical scanning. For example, the person being scanned could be placedupon a platform that was raised and lowered. This approach would beespecially convenient where a person entered the scanning area on onelevel and left it on another, an especially convenient arrangement,where, for example, an airline passenger mightenter at ground level andleave closer to boarding ramp level. Video storage and display unit 13then displays image 12 which, in this embodiment, is an image formed ofa sequence of horizontal lines as distinguished from the sequence ofvertical lines forming the image in FIG. 1.

Considering now specific parameters for a parcel examining system ofFIG. 1, such a system could examine parcels with dimensions up to 32 X20 X 16 inches provided that parcels with dimensions exceeding 20 inchesare oriented with their long axes parallel to their direction of traveland all parcels are guided close to detector 25 with a distance betweensource and detector of approximately 6 feet and the height of detector25 about 24 inches. Then the maximum distortion caused by differences inmagnification of the front and back surfaces of parcels will neverexceed i 19 percent from the average magnification and would occur onlyrarely.

" Objects with overall depths less than 20 inches along the direction ofthe scanning beam would have proportionately less distortion.

Resolution capabilities of l millimeter square are readily obtainablefor identification of most objects having characteristic dimensions ofseveral inches. With l mm resolution a 20 inch object could be coveredin 500 scans without gaps or overlap, larger parcels being covered bymore scans or greater spacing between scans. In either case the imagecould be displayed on a standard 5l2-line television monitor withnegligible loss of detail.

For a nominal conveyor speed of IO inch/second (250 mm/second), 250scans/second (or 4 milliscconds/scan) achieves l millimeter resolutionwhere each scan covers the full 24 inches height of the detector so thata 20-inch long parcel could be scanned in 2 seconds.

With X-ray tube 14 conventional and operating at moderate voltage andcurrent (60-100 kv, l0 ma,) it typically produces a flux at 6 feet (thedistance to detector 25) of millions of X-rays per mm per second. Afiltered tungsten target tube operating at 100 kv and I5 ma providestypically an X-ray flux at detector 25 of about 10 X-rays/mm /sec. witha broad energy spectrum extending from 20 to 100 kev. Generating 250,000resolution elements in 2 seconds results in each resolution elementbeing irradiated for about 2/250,000 seconds or 8 microseconds. With anX-ray flux at the detector 25 of 10 X-ray mm lsecond, each resolutionelement would (in the absence of an X-ray absorbing object) receiveabout X-rays per exposure. Taking into account the absorption by packingmaterial of low energy X-rays, 10-20 X-rays/resolution element wouldtypically be detected during a 2 second total exposure, about thestatistically significant number of X-rays required to distinguish whitefrom black in adjacent resolution elements so that the proposed 2-secondexposure time is appropriate to achieve l X 1 mm resolution.

A feature of the invention is that the X-ray detection process is ideal.The X-ray quantum efficiency of the detector 25 is close to percent.X-rays will produce output pulses several times larger thanphotomultiplier noise (dark) pulses so that the latter can be completelyeliminated by threshold discrimination. Moreover, since the detector canbe made very narrow, the background contribution from radiationscattered by a parcel is negligibly small so that the invention may useminimum X-ray dosage for I mm resolution, typically less than 0.003mrads per image compared with the daily dosage received from cosmic raysand naturally occurring radio activity of about 0.3 mrads and to thedosage required to expose X-ray film to a barely detectable 0.01 densityunit above background fog which requires at least 0.1 mrads. Thus, theinvention may be safely used for inspecting personnel and parcelswithout using harmful radiation levels.

Preferably the X-ray tube and associated power supply are conventional.Preferably X-ray tube 141's operable at variable voltages up to kv tooptimize image quality. Preferably X-ray tube 14 is operated watercooled with a peak voltage of 150 kv, peak current 5-l0 ma, a 100percent duty cycle, the power being at constant potential and the focalspot size of 0.4 mm, all these characteristics being readily available.

For the dimensions discussed above and a source spot size of 0.4 mm, aslit 21 width of 0.3 mm will provide l mm resolution. If disc 18 weremoved closer to detector 25, a wider slit could be used, but the discdiameter would increase proportionately. Conversely, a

smaller disc could be used if it were moved closer to X-ray tube 25, butthe slit size would have to be reduced. A 2-foot diameter disc with 0.3millimeter slits located midway between tube 14 and detector 25 is asatisfactory compromise between rotation of a larger disc at higherspeeds and fabrication of smaller slits. The slits themselves are shapedto collimate the beam along all pencils comprising the fan and may befabricated from tungsten inserts installed in the disc. The rate ofrotation of disc 18 is related to the time available for a fullexposure. For 500 scan lines in 2 seconds, disc 18 generating six scansper revolution rotates at 250/6 revolutions per second or 2,500 rpm, atrate readily achieved with standard motors.

A preferred form for detector 25 comprises a sodium iodide crystal thatdetects X-rays below 200 kev with 100 percent efficiency. Such adetector with dimensions l X l X 24 inches can be readily fabricatedfrom two or three shorter pieces of standard material. The energy ofeach X-ray interacting in sodium iodide is converted to lightsufficiently large to be easily detected by a photomultiplier. Byoptically coupling a 1 inch end window photomultiplier to each end ofthe sodium iodide crystal, there is complete and uniform lightcollection for X-ray interactions occurring at any position along thelength of the detector. The summed currents from the twophotomultipliers are proportional to the instantaneous X-ray fluxstriking the detector to produce an image signal analogous to anordinary video signal that, after amplification, may be stored and displayed by techniques known in the art.

It is preferred that the amplifier for the summed photomultiplier outputcurrents have a bandwidth from d-c to about lMl-lz to retain allinformation in a 500 scan exposure that may be completely transparent(or opaque) to X-rays for one parcel and may contain structure at thelimits of resolution (at 8 microseconds per resolution element) foranother parcel, preferably being low noise so as to not limit systemsensitivity and providing an output signal at high enough level to bestored. By utilizing as much amplification as practical from thephotomultiplier itself, many commercially available amplifiers, such ascommonly available oscilloscope preamplifiers are adequate.

In an actual working embodiment of the invention, the X-ray image wasreconstructed by employing suit ably triggered time-base units toprovide successive vertical sweeps, each slightly displaced from itsneighbor to produce a television-like raster scan and using the analogsignal derived from the photomultiplier tube to intensity modulate theCRT electron beam on a storage oscilloscope to produce an image that wasretained long enough for visual inspection.

lt is preferred to use a scan converted storage tube of a type wellknown in the art in order to produce a better image, such systems beingcommercially available and of the type used to convert slow radar scansto a continuously displayed television picture that is updated at everysuccessive radar scan.

The invention has numerous uses, including medical applications, and maytake many different forms. For example, there may be a number ofdetectors and fan beams arranged for providing a multiplicity ofscanning beams. Other techniques may be employed for providing thescanning beams of radiant. energy and for detecting transmitted and/orscattered energy.

There has been described a novel radiant energy imaging systemcharacterized by rellatively high resolution, low radiation dosage, easeof operation and numerous other features. It is evident that thoseskilled in the art may now make numerous uses and modifications of anddepartures from the specific embodiments described herein withoutdeparting from the inventive concepts. Consequently, the invention is tobe construed as embracing each and every novel feature and novelcombination of features present in or possessed by the apparatus andtechniques herein disclosed.

What is claimed is:

l. Radiant energy imaging apparatus comprising a source of a pencil beamof radiant energy,

radiant energy detecting means defining a curve in fixed relationshipto' said source,

means for scanning with said pencil beam said radiant energy detectingmeans along said curve to providean image signal representative of theradiant energy response of the medium in a region traversed by saidpencil beam along a path to said detecting means,

means for relatively displacing said region and an assembly comprisingsaid source and said detecting means to establish relative translatingmotion in a direction transverse to a line joining said source and saiddetecting means to produce a sequence of image signals representative ofthe radiant energy response of said region in two dimensions,

and means responsive to said image signals for producing an imagerepresentative of said response.

2. Radiant energy imaging apparatus in accordance with claim 1 whereinsaid radiant energy comprises X- rays.

3. Radiant energy imaging apparatus in accordance with claim ll whereinsaid source ofa pencil beam comprises,

a source of said radiant energy,

means for collimating said radiant energy into a slitlike beam,

and means defining an aperture for intercepting said slit-like beam toprovide said pencil beam,

said means for scanning comprising means for relatively moving saidaperture and said slit-like beam to effect said scanning.

4. Radiant energy imaging apparatus in accordance with claim 3 whereinsaid source of an uncollimated beam of said radiant energy comprises anX-ray tube,

said means for collimating comprises a plate of X-ray opaque materialformed with a slit of X-ray transparent material,

said means defining an aperture comprises a radial slit that is X-raytransparent in an X-ray opaque disc,

and said means for relatively moving comprises means for rotating saiddisc to move said radial slit along said first-mentioned slit.

5. Radiant energy imaging apparatus in accordance with claim 2 wherein,

said detecting means comprises means for converting incident X-rayenergy into light energy,

and photodetecting means responsive to the latter light energy forproviding an electrical image signal that is amplitude modulated inproportion to the instantaneous X-ray flux incident upon said detectingmeans.

6. Radiant energy imaging apparatus in accordance with claim and furthercomprising a television display system responsive to said image signalfor displaying a corresponding image.

7. Radiant energy imaging apparatus in accordance with claim 5 whereinsaid means is a crystal from the group consisting of sodium iodide andcesium iodide,

and said photodetecting means comprises photomultipliers at each end ofsaid crystal means.

8. Radiant energy imaging apparatus in accordance with claim 6 andfurther comprising means for relatively displacing a region to bescanned and said curve to display a two-dimensional image of the X-rayresponse of said region being scanned.

9. Radiant energy imaging apparatus in accordance with claim 7 andfurther comprising means for relatively displacing a region to bescanned and said curve to -provide a two-dimensional image signal of theX-ray response of said region being scanned.

10. Radiant energy imaging apparatus in accordance with claim 1 whereinsaid radiant energy comprises X- rays,

said source of a pencil beam comprising,

a source of said radiant energy,

said radiant energy detecting means defining a line,

means including a plate of X-ray opaque material formed with a linearslit of X-ray transparent material for collimating said radiant energyinto a slitlike beam embracing a plane substantially including said slitand said straight line, an X-ray tube comprising a source of theuncollimated beam of said radiant energy,

an X-ray opaque disc formed with at least one radial slit that is X-raytransparent,

and means for rotatably supporting said disc with its plane generallyperpendicular to the plane of said slit-like beam so that rotation ofsaid disc causes said radial slit to transmit contiguous portions ofsaid slit-like beam to said detecting means to effectively provide saidpencil beam scanning said straight line from one end to the other.

11. Radiant energy imaging apparatus in accordance with claim 10 whereinsaid means for relatively displacing comprises means for translating anobject to be imaged transverse to and across said plane substantiallyincluding said straight line and said slit-like beam.

12. Radiant energy imaging apparatus in accordance with claim 10 whereinsaid means for relatively displacing comprises means for moving saidsource and said detecting means together while said region remainsstationary.

1. Radiant energy imaging apparatus comprising a source of a pencil beamof radiant energy, radiant energy detecting means defining a curve infixed relationship to said source, means for scanning with said pencilbeam said radiant energy detecting means along said curve to provide animage signal representative of the radiant energy response of the mediumin a region traversed by said pencil beam along a path to said detectingmeans, means for relatively displacing said region and an assemblycomprising said source and said detecting means to establish relativetranslating motion in a direction transverse to a line joining saidsource and said detecting means to produce a sequence of image signalsrepresentative of the radiant energy response of said region in twodimensions, and means responsive to said image signals for producing animage representative oF said response.
 2. Radiant energy imagingapparatus in accordance with claim 1 wherein said radiant energycomprises X-rays.
 3. Radiant energy imaging apparatus in accordance withclaim 1 wherein said source of a pencil beam comprises, a source of saidradiant energy, means for collimating said radiant energy into aslit-like beam, and means defining an aperture for intercepting saidslit-like beam to provide said pencil beam, said means for scanningcomprising means for relatively moving said aperture and said slit-likebeam to effect said scanning.
 4. Radiant energy imaging apparatus inaccordance with claim 3 wherein said source of an uncollimated beam ofsaid radiant energy comprises an X-ray tube, said means for collimatingcomprises a plate of X-ray opaque material formed with a slit of X-raytransparent material, said means defining an aperture comprises a radialslit that is X-ray transparent in an X-ray opaque disc, and said meansfor relatively moving comprises means for rotating said disc to movesaid radial slit along said first-mentioned slit.
 5. Radiant energyimaging apparatus in accordance with claim 2 wherein, said detectingmeans comprises means for converting incident X-ray energy into lightenergy, and photodetecting means responsive to the latter light energyfor providing an electrical image signal that is amplitude modulated inproportion to the instantaneous X-ray flux incident upon said detectingmeans.
 6. Radiant energy imaging apparatus in accordance with claim 5and further comprising a television display system responsive to saidimage signal for displaying a corresponding image.
 7. Radiant energyimaging apparatus in accordance with claim 5 wherein said means is acrystal from the group consisting of sodium iodide and cesium iodide,and said photodetecting means comprises photomultipliers at each end ofsaid crystal means.
 8. Radiant energy imaging apparatus in accordancewith claim 6 and further comprising means for relatively displacing aregion to be scanned and said curve to display a two-dimensional imageof the X-ray response of said region being scanned.
 9. Radiant energyimaging apparatus in accordance with claim 7 and further comprisingmeans for relatively displacing a region to be scanned and said curve toprovide a two-dimensional image signal of the X-ray response of saidregion being scanned.
 10. Radiant energy imaging apparatus in accordancewith claim 1 wherein said radiant energy comprises X-rays, said sourceof a pencil beam comprising, a source of said radiant energy, saidradiant energy detecting means defining a line, means including a plateof X-ray opaque material formed with a linear slit of X-ray transparentmaterial for collimating said radiant energy into a slit-like beamembracing a plane substantially including said slit and said straightline, an X-ray tube comprising a source of the uncollimated beam of saidradiant energy, an X-ray opaque disc formed with at least one radialslit that is X-ray transparent, and means for rotatably supporting saiddisc with its plane generally perpendicular to the plane of saidslit-like beam so that rotation of said disc causes said radial slit totransmit contiguous portions of said slit-like beam to said detectingmeans to effectively provide said pencil beam scanning said straightline from one end to the other.
 11. Radiant energy imaging apparatus inaccordance with claim 10 wherein said means for relatively displacingcomprises means for translating an object to be imaged transverse to andacross said plane substantially including said straight line and saidslit-like beam.
 12. Radiant energy imaging apparatus in accordance withclaim 10 wherein said means for relatively displacing comprises meansfor moving said source and said detecting means together while saidregion remains stationary.